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Acoustics, Ultrasound and Vibration Capabilities

NMI’s acoustics, ultrasonics and vibration group is responsible for conducting research into and maintaining Australia’s primary standards. It also provides high-accuracy NATA-accredited calibration services in a wide range of areas including:

Contact us if you have a specialised or unusual instrument not mentioned here.

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Before consigning an instrument for calibration always consult us to discuss your requirements. For further information contact calibration.coordinator@measurement.gov.au.

microphonesMicrophones

Capacitor microphones including the laboratory-standard one-inch LS1P and half-inch LS2P as defined in IEC 61094-1:2000, are calibrated by 2-port pressure reciprocity over the frequency range 31.5 Hz to 31.5 kHz (depending on the type of microphone). Uncertainties are dependent on frequency, but are as low as 0.04 dB.

Measurements of one-inch microphones may also be made using 3-port reciprocity at the preferred frequency of 250 Hz for a single point reference, or at the secondary frequencies of 500 Hz and 1000 Hz. The uncertainty of a reciprocity calibration of a test microphone with the primary standard set is generally 0.05 dB depending on the microphone type.

Microphones that cannot be calibrated by reciprocity can be calibrated by comparison with a standard reference microphone in an acoustic coupler. The frequency range is from 20 Hz to 2 kHz for one-inch and from 20 Hz to 25 kHz for half-inch microphones and the measurement uncertainty is generally less than 0.1 dB depending on the microphone type.

The electrostatic actuator also provides a means of measuring microphone frequency response. The technique uses an electrostatic force between the diaphragm and a grid placed over it. The electrostatic force simulates the action of a sound field closely approximating a pressure response. The technique is normally restricted to one-inch or half-inch microphones where it is possible to gain access to the diaphragm. Special adaptors are used to also allow calibration of other types including the IEC LS1P or LS1F and LS2P or LS2F types. Electrostatic actuator calibrations are normally performed at third-octave frequencies between 20 Hz and 20 kHz but may be extended to 100 kHz for quarter-inch microphones. The uncertainty would generally be less than 0.2 dB at frequencies up to 20 kHz but will depend on type.

Low-frequency microphones can be calibrated at frequencies between 1 and 250 Hz by comparison with a low-frequency reference microphone in a special coupler. This low-frequency reference microphone is calibrated using the electrostatic actuator technique. An assurance test is available to check the function of low-frequency microphones at a set number of frequencies and amplitudes. Checks of amplitude linearity may be carried out up to 140 dB SPL, usually at 31.5 Hz.

For free-field measurements, NMI maintains the free-field primary reference standard consisting of an LS2P capacitor microphone. This microphone was initially calibrated by the free-field reciprocity technique as well as by pressure reciprocity, and its stability is periodically established by calibration using pressure reciprocity. Free-field calibration of microphones is carried out in an anechoic chamber between 31.5 Hz and 20 kHz by substitution either with a working reference microphone which is traceable to the free-field primary reference standard or directly using the free-field primary reference standard itself.

pistonphonesPistonphones and Acoustic Calibrators

Pistonphones and acoustic calibrators provide sound pressure fields for secondary calibration and checking of the output of microphones. Microphones are inserted into the cavity within the calibrator and the output of the microphone is measured. Pistonphones and calibrators are normally calibrated using laboratory standard microphones which in turn have been calibrated by the reciprocity technique. An insert-voltage technique is used whereby the electrical output of the microphone, when used in the calibrator, is compared to an accurately measured ac voltage applied in such a way as to simulate the microphone output. The known open-circuit sensitivity of the microphone allows a value for the output of the calibrator under test to be established. For best results it is recommended that the client advise NMI of the type of microphone that will be used with the device.

For pistonphones and acoustic calibrators that use other types of microphones, a substitution technique using a variable sound source is used. Using laboratory standard microphones calibrated by reciprocity, and an insert-voltage technique, the variable source is set up to the nominal value of the device under test. The comparison is made using the microphone type specified for the device. For accurate results, it is recommended that the client submit an instrument with which the pistonphone or acoustic calibrator is intended to be used. This will be used as the detector/microphone in the substitution process.

Use of a pistonphone or calibrator on an instrument for which it was not designed or for which there is no calibration may lead to error.

The calibrators are tested with reference to AS IEC 60942:2004.

sound level metersSound Level Meters and Filters

Sound level meters are calibrated according to the tests and tolerances described in AS 1259.1:1990 (non-integrating) and AS 1259.2:1990 (integrating-averaging) or IEC 61672-3:2006. In addition, tests devised by NMI are used for integrating/averaging sound level meters which may include a statistical analyser function. The list of functions and characteristics tested are based on OIML R 58 and OIML R 88 and may include:

  • differential level linearity
  • level range control accuracy
  • fast, slow, impulse and peak detector-indicator characteristics
  • RMS detector performance
  • frequency response of weighting networks
  • whole meter body frequency response in a free-field environment
  • absolute sensitivity under reference conditions
  • overload indicator
  • in addition, the various integrating/averaging facilities (e.g. SEL, Leq, Lx) can be tested

Most of these tests are performed using electrical signals input via a substitute 'dummy' microphone. AS 1259.1:1990 and IEC 61672-3:2006 specify that an alternative input to the microphone be available on the sound level meter for this purpose.

Band-pass octave and one-third octave filters intended for the analysis of sound and vibration are calibrated according to the specifications in AS/NZ 4476:1997 and IEC 1260.

Resistive attenuators and sine generators for use in acoustic measurements can also be tested over a range of attenuations, from 0 to 120 dB. It is usual to make these tests at three frequencies in the audible range, e.g. 31.5 Hz, 1 kHz and 12.5 kHz. The preferred impedance is 600 Ω unbalanced.

blast and noise monitorBlast and Noise Monitors

Low frequency microphones used to monitor blast overpressures can be calibrated by comparison with suitable reference microphones in a special coupler capable of completely enclosing both the test and reference devices. Such microphones can be examined at a suitable number of frequencies in the range 1 to 250 Hz and at selected amplitudes.

Instrumentation and special outdoor microphones used for environmental noise monitoring are calibrated between 31.5 Hz and 20 kHz by comparison with a free-field reference microphone. NMI's anechoic chamber is used for these free-field measurements.

Artificial Mastoids

cross section of a human headHumans hear sounds not only by the transmission of sound through the ear canal, but also by transmission of vibrations through the skull. For people with hearing problems caused by middle- or outer-ear dysfunction, hearing aids that vibrate the mastoid bone (a bony structure just behind the ear) which in turn conducts the sound directly to the inner ear, may be used.

The hearing acuity of people, and how much sound is transmitted to the ear via the ear canal and via the bones of the skull, may be measured using audiometric methods. Bone conduction audiometers are used in which mechanical vibrations generated by an electromechanical transducer (bone vibrator) applied to the skin of the bone in which the ear is embedded (the mastoid). The vibration levels applied to a person can only be known by calibrating the bone vibrator. This is performed using an artificial mastoid.

These hearing aids (bone vibrators) are calibrated by measuring their output and the forces they apply to the human head by using device called an artificial mastoid. This is a mechanical simulation of the mastoid bone of the human head. It comprises curved multiple layers of visco-elastic materials to simulate the human structure, with an embedded accelerometer, all mounted in a case of weight 3.5 kg, approximately the mass of a human head. The sensitivity and mechanical impedance of the artificial mastoid must be known accurately. Calibration of artificial mastoids is performed by measuring their mechanical impedance which is a measure of the structure’s ability to absorb a vibration.

The mechanical impedance of artificial mastoids is influenced by environmental conditions, especially temperature, and so regular calibration is necessary to ensure their properties continue to be within the tolerances of IEC 60318-6, and that bone vibrator hearing aids are correctly adjusted to maximise the hearing of the wearer and minimise discomfort. NMI can now perform these calibrations, which provide the mechanical impedance (in units of N/(m.s-1)) and force sensitivity (in V/N) of an artificial mastoid over the frequency range 125 to 8000 Hz, with uncertainties in the range 0.4 to 0.5 dB.

accelerometerAccelerometers

NMI is mainly concerned with the calibration of reference grade accelerometers and accelerometers that are used for precise or critical measurements of vibration. Most popular makes and models of accelerometers are catered for. A calibration of an accelerometer is performed either by comparison with the response characteristics of a calibrated NMI reference standard, or by applying absolute measurement techniques through laser interferometry. Typically an accelerometer is calibrated with its associated charge amplifier or power supply, comparing its voltage output with the acceleration measured with our standard or laser interferometer, for a number of frequencies and acceleration amplitudes. The normal units of this type of calibration are millivolts per metre per second squared (mV/(m/s2)) or millivolts per standard gn (mV/gn, where gn = 9.806 65 m/s2). Charge accelerometers can also be calibrated without a conditioning amplifier, and in this case the sensitivity of the accelerometer is expressed in picocoulombs per metre per second squared or per gn (pC/(m/s2) or pC/gn).

vibration meterVibration Meters and Monitors

A vibration meter is usually regarded as any vibration measurement system that provides a direct output of the vibration level monitored on an instrument display, oscilloscope or any other means of presenting information to an operator. A vibration monitor is usually equipment that logs readings of vibration levels over long periods of time. Calibration of these instruments is generally carried out in a manner similar to that of calibrating an accelerometer as described above, with a reading made from the vibration meter or monitor for each calibration frequency and amplitude.

geophoneGeophones and Velocity Transducers

Horizontal and vertical uni-directional, and omni-directional velocity transducers and geophones can be calibrated at frequencies from near 0 to 250 Hz. The limitation of mass for vertically oriented transducers is 0.5 kg and horizontally oriented transducers is 5 kg. Velocity transducers are usually calibrated against NMI reference accelerometers and can be done over wider frequency ranges. The normal calibration units of geophones and other velocity transducers are millivolts per millimetre per second (mV/(mm/s)). Calibration of accelerometers with charge amplifiers set to provide an output proportional to velocity, may be calibrated as an accelerometer.

Shock and impulse systemShock and Impulse Systems

Calibration and verification of compliance with the requirements of SAE J211b, SAE J211/1 and ISO 6487 for shock measuring systems, which include accelerometers and filtering units, is available. Calibration of shock and impulse systems is performed using steady state excitation and compared with an NMI standard. Nominal units of shock systems are mV/(m/s2) or mV/gn and relative response is given in decibels (dB). Measurement of instrumented hammers can also be undertaken.

Portable shakerPortable Calibration Exciters and Shakers

Portable calibration exciters and shakers can be calibrated by comparison with an NMI reference accelerometer. A typical calibration would involve ascertaining the excitation amplitude and drive frequency of the shaker for up to two or three external mass loadings (normally 20, 60 and 90 g). Results are usually quoted as drive excitation in peak acceleration (m/s2), velocity (mm/s) and displacement peak (µm).

Electronic Filters and Conditioning Amplifiers

Normally calibrations are performed on complete systems including transducers, amplifiers and any associated filters. However, a charge amplifier or filter can be calibrated independently of a transducer. This is typically done where a number of settings or configurations for an amplifier (such as range and filter settings) are required to be used with a single transducer. In this case a normal calibration of the transducer with the amplifier is performed for one setting of the amplifier/filter. The amplifier is then calibrated independently of the transducer for one or more additional configurations. Charge amplifiers are calibrated by comparing a known charge, from a calibrated standard capacitor, to the amplifier's output. The units of a charge amplifier calibration are typically millivolts per pico-coulomb (mV/pC). For independent calibration of electrical filters, the results are usually given as a ratio, in decibels (dB), of the input voltage compared to the output voltage.

Ultrasonic Power Meters and Ultrasonic Transducers

Whilst guidelines exist for the medical use of ultrasound, the emphasis of responsibility is now placed on the operator to ensure that ultrasound is used safely. Generally, low powers are used for medical diagnosis, and high for medical therapy, but in both cases the power level needs to be known accurately for effective and safe treatment. The range of powers used varies from tens of milliwatts to tens of watts per square centimetre.

Organisations checking these transducers will usually have an ultrasonic power meter which measures the output powers, typically from tens of milliwatts to tens of watts. Such meters and standard transducers should be calibrated at manufacture and regularly during their lifetime. NMI offers this service for both transducers and power meters, measuring powers of 10 mW to 20 W in the frequency range 0.5 to 25 MHz. Uncertainties are typically 3 to 6% for transducers and 5 to 8% for power meters, both well within the guidelines, for example, specified in IEC 61689 of 15% for ultrasonic physiotherapy devices. NMI is the first laboratory in Australia to offer this NATA-accredited calibration.